1. Technical Field
This invention relates to radiation therapy, and more particularly to a method and apparatus for controlling and verifying the shape and location of a radiation beam during radiation therapy.
2. Discussion
Radiation therapy deals with the treatment of disease with ionizing radiation. The diseases most commonly treated in this manner are cancer and allied diseases. In cancer therapy the objective is to destroy a tumor without causing irreparable radiation damage in normal body tissues that must of necessity be irradiated in the process of delivering a lethal dose to the tumor. This applies particularly to important normal structures in the vicinity of the tumor. Thus, the relative radio-sensitivity of the tumor with respect to these normal structures is an important factor determining the success of the treatment.
The optimal differential between the effect on the tumor and the effect on normal tissues of the patient as a whole results from the proper adjustment of many treatment factors. The time factor is very important. This involves the administration of the therapeutic dose in one of three ways: by one short treatment; by protraction as continuous irradiation over a long time; or by fractionation in small repeated doses. The importance of the distribution of the radiation in the patient's body is obvious. Ideally, only the tumor should be irradiated but this is normally impossible, because in general there is adjacent normal tissues overlaying or underlaying the tumor volume.
One technique for minimizing the exposure of normal tissue is to control the precise shape of the radiation beam. This can be done by simulating the radiation therapy in a simulator prior to actual radiation treatment. In one conventional technique, a patient is placed in a simulator and a light in the simulator simulates the rectangular beam shape of the radiation field on the patient. This beam axis location as well as field corners can then be drawn on the patient's skin to aid the set-up on radiation therapy apparatus. In addition, an X-ray can be taken of the target area on the patient while in the simulator. The desired field shape surrounding the tumor is then drawn (typically by hand) on the X-ray film. The X-ray film with the hand drawn field shape is then used to manufacture a set of field shaping blocks which are placed in a tray in the radiation therapy apparatus to control the shape of the radiation beam to the desired shape. For example, the blocks may be made of molded cerrobend. When the patient is placed in the radiation therapy apparatus, the rectangular field shape markings on the patient can be lined up with a field shape light simulating the actual beam position. Then, the blocks conforming to the particular desired field shape are installed and the radiation beam is administered.
One problem with this conventional technique is the length of time that it takes to produce the (cerrobend) blocks, which is usually a matter of hours or days. Storage of the bulky blocks is also a problem. Another problem with this technique is that only the rectangular field shape is drawn on the patient and verified in the treatment machine, but not the actual irregular field shape. This limits the accuracy of positioning of the patient with respect to the desired field. One approach sometimes used to overcome this particular limitation is to return the patient to the simulator after the cerrobend blocks are manufactured. With the blocks placed in the simulator the exact desired irregular field shape is then drawn on the patient. Then, when the patient is placed in the treatment apparatus, with the same blocks installed, the field projection light will project the field shape through the blocks onto the patient and by using the marks on the patient, the patient can be precisely positioned. However, while this approach improves accuracy, it increases rather than decreases the number of steps required to accomplish the desired treatment.
An alternative approach to controlling the radiation beam which is gaining wide acceptance is the use a device known as a multi-leaf collimator. The multi-leaf collimator consists of opposing arrays of narrow tungsten leaves placed in front of the radiation beam. By driving each leaf into different positions, virtually any desired field shape can be achieved in radiation therapy. It is expected that all new medical linear accelerators will eventually include multi-leaf collimators.
A major drawback with the use of multi-leaf collimators is that there is no multi-leaf collimator on the simulator, and thus the field shape information cannot be drawn on the patient during simulation. Instead, the rectangular beam shape is the only useful shape that can be drawn on the patient. As a result, without additional marks, it is difficult to align the patient and treatment beam correctly for the radiation treatment. One obvious approach to solve this problem would be to use the a multi-leaf collimator inside the simulator. However, the complexity and cost of the multi-leaf collimator generally prevents such an approach. While less expensive simulator multi-leaf collimators using plastic leaves have been proposed, the cost and complexity of such an apparatus still are considered prohibitive for use in simulation.
Another problem with the multi-leaf collimator systems is the inability to verify the correct leaf position just prior to treatment. That is, there is currently no way to insure that all of the leaves are properly functioning and in proper position because there is not an effective technique for marking the actual field shape on the patient's skin with multi-leaf collimator systems. One possible way to do this is to transmit a low dose radiation test beam through the collimator and patient onto an X-ray filter. Unfortunately, the low dosage of the test beam results in an unclear image which does not permit accurate positioning of the patient and/or beam.
Thus, it would be desirable to provide a means for improving the positioning and verification of beam shape and location in radiation therapy. It would also be desirable to provide such a system which can provide accurate verification of beam position that requires only a single visit by the patient. Further, it would be desirable to provide a method for simulating beam shape and placement which is compatible with multi-leaf collimator systems. It would also be desirable to provide such a system which provides for the marking of the actual beam treatment pattern on the skin of the patient and which also is compatible with multi-leaf collimator systems. Also it would be desirable to provide a system which can verify the correct leaf placement in a multi-leaf collimator system.